History of Flight
On May 30, 2023, at approximately 1115 Pacific daylight time, a Cessna 210B (registration N9574X) sustained substantial damage during an accident near Tacoma, Washington. The aircraft was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The flight instructor and student pilot were not injured.
According to the flight instructor, she intended to perform a practice landing at Boeing Field in Seattle, Washington. While on the downwind leg of the traffic pattern, she completed the landing checklist but encountered an issue with the landing gear. The landing gear did not fully extend and appeared stuck in a transitional position. The instructor moved the gear selector back to the gear-up position but observed no landing gear movement. Returning the selector to the gear-down position also produced no movement. Suspecting a hydraulic system problem, she attempted to extend the hydraulically actuated wing flaps, which was unsuccessful.
The flight instructor requested a low approach over the runway so air traffic controllers could visually verify the landing gear position. Controllers reported the gear appeared stuck in transition. The instructor then diverted to a practice area to troubleshoot. She used the emergency backup hand-pump without success. She contacted a maintenance technician who attempted to troubleshoot the landing gear, but the situation did not change. Subsequently, she diverted to Tacoma Narrows Airport in Tacoma, Washington, and performed several low passes over mechanics on the ground who assessed the condition of the landing gear.
After approximately 1.5 hours of troubleshooting, the flight instructor decided to land with the gear partially extended. The airplane touched down on the runway centerline with the nose gear locked in place, which allowed for limited turning ability. The airplane came to rest in the grass off the left side of the runway. The left horizontal stabilizer was damaged during the accident sequence.
Aircraft Information
A comparison of the Airworthiness Directive (AD) compliance listing in the logbook against a list of ADs applicable to the airplane’s serial number revealed that all pertinent ADs had been endorsed as complied with. This included AD 76-04-01, which was shown as complied with in August 1985.
Additional Information
Actuator
Cessna sourced actuators from two companies: Electrol Inc. and Ozone. The Electrol Inc. unit used a snap ring to hold the end plug in place, with a cover plate positioned over the end plug to pull it out to the snap ring and secure it. The Ozone design did not use a snap ring; instead, the end cap was screwed in, which was considered more robust. A Service Bulletin was issued to address inspections of these actuators.
Prior Accidents
This accident was at least the fourth known failure of this type of actuator. The National Transportation Safety Board examined actuators from four accidents, including this one:
- Cessna 210 accident in Juneau, Alaska (2015): Actuator P/N EA1614, S/N 267 (Electrol Inc.)
- Cessna 210C accident in Juneau, Alaska (2018): Actuator P/N EA1614…, S/N 275… (most of placard obliterated)
- Cessna 210B accident in Clinton, Arkansas (2022): Actuator P/N EA161402, S/N 190 (Cox Airparts)
The fractures from all these accidents were identical. The location, size, and physical features of the cracks were consistent and included fatigue cracks initiating from the retainer clip grooves on the interior of the actuator housings that propagated outward into the housing bodies. When the crack grew large enough, the housing fractured, relieving hydraulic pressure and causing the actuator to fail in service.
Maintenance Inspections
Numerous service bulletins and airworthiness directives were issued on these hydraulic actuators over several decades, including:
- Cessna Service Letter 67-16 (March 28, 1967) on replacement
- Cessna Service Letter SE69-17 (September 16, 1969) on field repair
- Cessna Service Letter SE75-21 (October 3, 1975) on modification
- Airworthiness Directive 76-04-01 (January 27, 1977) on repair/replacement
- Cessna Service Bulletin SEB01-2 Rev 2 (June 4, 2007)
These notices did not cover the actuators previously examined by the NTSB, though AD 76-04-01 covered the part in this investigation, having P/N 1280501-1. The markings on the actuator were consistent with it having been refurbished.
Tests and Research
A postaccident examination of the landing gear was performed by a certified airframe and powerplant mechanic. He stated that the landing gear doors were locked in the open position. Removing the panels revealed that the left main gear rotary actuator was cracked and leaking hydraulic fluid.
The Electrol Inc. actuator, part number (P/N) 1280501-1, was sent to the NTSB Materials Laboratory for examination. The examination revealed a crack through the barrel, with circumferential and longitudinal sections. The crack initiated at the retaining ring groove, where corrosion pits were present, and propagated via fatigue circumferentially. The longitudinal portion of the crack, which extended through more than half the actuator barrel's thickness, displayed signatures consistent with tensile overstress fracture. The actuator was composed of aluminum alloy, and its hardness and conductivity were consistent with expected material properties.
Scanning electron microscopy (SEM) examination confirmed that the fatigue cracking originated at corrosion pits along the ring groove. As demonstrated by striation features, the fatigue crack propagated circumferentially around the barrel until it reached a critical size, and then partially fractured longitudinally from overstress. Corrosion pits were present at the fatigue crack initiation points along the groove.
Energy dispersive x-ray spectroscopy (EDS) found that the retaining clip groove contained remnants of cadmium plating from the clip and confirmed the presence of aluminum oxide at the crack initiation sites. These findings were consistent with fatigue failure due to the presence of corrosion pits and the progressive propagation of cracks under cyclic stress, leading to the actuator's failure and the release of hydraulic fluid.